The detection and identification of micro-organisms is very important in several fields, such as diagnosis of infectious disease, tracing sources of outbreaks of disease or contamination, detecting and tracing the spread of antibiotic resistance etc.
Many methods of detecting and identifying micro-organisms have been derived in recent years, employing sophisticated techniques, such as PCR and the like. These are frequently rapid and extremely sensitive. However, there is still a need for more traditional methods involving the culturing of organisms on media (especially solid media). These traditional methods have the great advantage that they result in isolation of organisms of interest in viable form, which can then be sub-cultured into other media and/or further analysed.
It is well-known to include in such media compounds which may allow or promote the growth of certain micro-organisms whilst preventing or inhibiting the growth of other micro-organisms. Such media are said to be “selective”. For example, certain antibiotics may be incorporated into media to allow the growth of micro-organisms which are sufficiently resistant to the antibiotic(s) employed.
It is also well known to include in media (which may or may not be selective) substances which allow different (possibly closely-related) micro-organisms to be distinguished. Such media may be described as “differential media”.
One such differential medium is disclosed in U.S. Pat. No. 5,716,799 (Rambach). The medium disclosed in that document comprises: at least one chromogen which is a substrate for an enzyme of the particular micro-organism to be identified, together with a carbohydrate at high concentration (10-30 gms/liter). The medium is such that, in the presence of the micro-organism of interest, the chromogen is hydrolysed to release a chromophore with a “derived colour” which is different to the basic colour of the chromophore in a “standard” medium. (A “standard” medium is defined in that document as being “any ordinary identification medium in which the carbohydrate has a simple function of a carbon source, at very low or even zero concentrations”.)
Thus, the teaching of U.S. Pat. No. 5,716,799 is that it is an essential feature of the invention described therein that the medium contains a high concentration (i.e. 10-30 gms/liter) of carbohydrate. A medium in accordance with U.S. Pat. No. 5,716,799 is commercially available from Becton Dickinson, USA and is known as CHROMagar™ Candida. 
The medium disclosed in U.S. Pat. No. 5,716,799 is useful in discriminating between different yeasts of the genus Candida. In particular, there is a clinical significance attached to the ability of distinguishing between Candida albicans and other species of Candida. C. albicans is a frequent human pathogen, but other Candida species may also be present. Also, different Candida species have different susceptibilities to antifungal therapy, so it is important to determine if an organism of the genus Candida is C. albicans or some other Candia species.
U.S. Pat. No. 5,534,415 (Orenga) also discloses an identification medium, specifically a medium for the selective detection of C. albicans. The medium disclosed therein comprises a chromogenic or fluorogenic substrate capable of being hydrolysed by a hexosaminidase enzyme “associated” with C. albicans, and at least one hexosamine-containing “activator” compound (different to the substrate) present at about 1 gm/liter. A medium in accordance with U.S. Pat. No. 5,534,415 is commercially available (from bioMérieux, France) and is known as Candida ID. Hydrolysis of the chromogen in the medium does not give rise to a derived colour (that is, the chromophore has the same colour in the Candida ID medium as it does in a standard medium).
Willinger et al (2001 J. Clin. Microbiol. 39, 3793-3795) conducted a trial to compare the performance of CHROMagar™ Candida and Candida ID, using nearly 600 clinical specimens. On CHROMagar™ Candida plates, colonies of C. albicans were green, whilst other colonies of other Candida species were pink, violet or white. On Candida ID plates, C. albicans colonies appeared blue, whilst other Candida species were pink or white.
Willinger et al found that, for about 50% of specimens, identification of C. albicans was more rapid on Candida ID plates than on CHROMagar™ Candida plates, the blue colour on Candida ID plates typically appearing after 24 hours' incubation at 35° C., whilst the green colour of C. albicans colonies on CHROMagar™ Candida plates often was not apparent until incubation for about 48 hours. This was expected, in that the Candida ID medium contains a hexosanine activator which, it is presumed, accelerates induction of the hexosaminidase enzyme which acts on the chromogen, whilst CHROMagar™ Candida medium does not contain any such activator. However, they also found that “Candida ID is not as good as CHROMagar™ Candida in detecting polyfungal specimens”.
In addition, CHROMagar™ Candida is superior to Candida ID in terms of its ability to allow different Candida species to be distinguished. Candida ID medium is very effective at distinguising between C. albicans and other species of Candida but cannot, for example, allow for efficient differentiation between C. tropicalis and other non-albicans species. CHROMagar™ Candida, in contrast, allows for the identification of a number of Candida species, including C. tropicalis, since various species give different “derived” colour changes on the medium. Candida ID medium does not give a “derived” colour chromophore which is different to that obtained with a basic medium.
The present invention is concerned with media for micro-organisms, especially differential media and, in particular, a medium which allows for discrimination between different species of Candida. 
The content of all publications mentioned in this specification is specifically incorporated herein by reference.